Non-destructive inspection (NDI) of structures involves thoroughly examining a structure without harming the structure or requiring its significant disassembly. Non-destructive inspection is typically preferred to avoid the schedule, labor, and costs associated with removal of a part for inspection, as well as avoidance of the potential for damaging the structure. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to or flaws in the structure. Inspection may be performed during manufacturing of a structure and/or once a structure is in-service. For example, inspection may be required to validate the integrity and fitness of a structure for continued use in manufacturing and future ongoing use in-service. However, access to interior surfaces is often more difficult or impossible without disassembly, such as removing a part for inspection from an aircraft.
Among the structures that are routinely non-destructively tested are composite structures, such as composite sandwich structures and other adhesive bonded panels and assemblies, such as hat stringers or hat stiffeners made from carbon fiber reinforced and graphite epoxy (Gr/Ep) materials and co-cured or co-bonded hat stringers. In this regard, composite structures are commonly used throughout the aircraft industry because of the engineering qualities, design flexibility and low weight, such as the stiffness-to-weight ratio. As such, it is frequently desirable to inspect composite structures to identify any flaws, such as cracks, voids or porosity, which could adversely affect the performance of the composite structure.
Various types of sensors may be used to perform non-destructive inspection. One or more sensors may move over the portion of the structure to be examined, and receive data regarding the structure. For example, a pulse-echo (PE), through transmission (TT), or shear wave sensor may be used to obtain ultrasonic data, such as for thickness gauging, detection of laminar defects and porosity, and/or crack detection in the structure. Resonance, pulse echo or mechanical impedance sensors may be used to provide indications of voids or porosity, such as in adhesive bondlines of the structure. High resolution inspection of aircraft structure are commonly performed using semi-automated ultrasonic testing (UT) to provide a plan view image of the part or structure under inspection. For example, solid laminates may be inspected using one-sided pulse echo ultrasonic (PEU) testing and composite sandwich structures may be inspected using two-sided through-transmission ultrasonic (TTU) testing. In pulse echo ultrasonic (PEU) testing, ultrasonic sensors, such as ultrasonic transducers, are positioned adjacent to or near one surface of the structure to be inspected. For example, the PEU transducer transmits an ultrasonic signal into the structure under inspection and receives the reflection of the ultrasonic signal from the structure. In through-transmission ultrasonic inspection, paired ultrasonic sensors such as transducers, or transducer and a receiver pairings, are positioned facing the other but contacting opposite sides of the structure. An ultrasonic signal is transmitted by at least one of the transducers, propagated through the structure, and received by the other transducer. Data acquired by sensors, such as PEU and TTU transducers, is typically processed by a processing element, and the processed data may be presented to a user via a display. A data acquisition board and data handling software may be used for collection and display of inspection data, such as displaying the data on a computer monitor as an image representation of the structure under inspection, such as a hat stringer, supplemented with corresponding color and/or graphical data of the inspection to permit examination by a qualified inspector.
Non-destructive inspection may be performed manually by technicians who typically move an appropriate sensor over the structure. Manual scanning requires a trained technician to move the sensor over all portions of the structure needing inspection. Manual scanning typically involves the technician repeatedly moving a sensor side-to-side in one direction while simultaneously indexing the sensor in another direction. In addition, because sensors typically do not associate location information with the acquired data, the same technician who is manually scanning the structure must also watch the sensor display while scanning the structure to determine where the defects, if any, are located in the structure. The quality of the inspection, therefore, depends in large part upon the technician's performance, not only regarding the motion of the sensor, but also the attentiveness of the technician in interpreting the displayed data. Thus, manual scanning of structures is time-consuming, labor-intensive, and prone to human error.
Semi-automated inspection systems have also been developed. For example, the Mobile Automated Scanner (MAUS®) system is a mobile scanning system that generally employs a fixed frame and one or more automated scanning heads typically adapted for ultrasonic inspection. A MAUS system may be used with pulse-echo, shear wave, and through-transmission sensors. The fixed frame may be attached to a surface of a structure to be inspected by vacuum suction cups, magnets, or like affixation methods. Smaller MAUS systems may be portable units manually moved over the surface of a structure by a technician.
Automated inspection systems have also been developed. For example, the Automated Ultrasonic Scanning System (AUSS®) system is a complex mechanical scanning system that may employ through-transmission ultrasonic inspection. An AUSS system can also perform pulse echo inspections, and simultaneous dual frequency inspections. The AUSS system has robotically controlled probe arms that may be positioned, for example, for TTU inspection proximate the opposed surfaces of the structure undergoing inspection with one probe arm moving an ultrasonic transmitter along one surface of the structure, and the other probe arm correspondingly moving an ultrasonic receiver along the opposed surface of the structure. To maintain the ultrasonic transmitter and receiver in proper alignment and spacing with one another and with the structure undergoing inspection, a conventional automated inspection system may have a complex positioning system that provides motion control in numerous axes, such as the AUSS-X system which has motion control in ten axes. Automated inspection systems, and like robotics, however, can be prohibitively expensive. Further, orienting and spacing sensors with respect to the structure, and with respect to one another for TTU inspection, may be especially difficult in conjunction with structures with non-planar shapes, such as the inspection of curved structures and hat stringers. Also, conventional automated scanning systems, such as the AUSS-X system, may require access to both sides of a structure which, at least in some circumstances, will be problematic, if not impossible, particularly for very large or small structures. Furthermore, scanning systems inspect limited areas up to a few meters square.
Accessibility to the structure requiring inspection and to particular features is also an important consideration. Access may be so limited that manual inspection or automated inspection is not possible. For example, the inside of a hat stringer of the fuselage of an aircraft has limited access for inspection, especially far from an end.